The invention relates to apparatus for growing and pulling a crystal from a melt in a crucible with a reelable pulling member having a crystal holder fastened on one end and, more particularly, the crystal holder.
The crystal-pulling methods carried out with such apparatus are often called Czochralski methods. The melt is usually a semiconductor material such as silicon.
An apparatus of the type referred to above is known from published German patent application DOS No. 31 16 916. It discloses a crystal holder of one-piece construction and relatively-short overall length fastened to a reelable pulling member which, preferably, is a steel rope.
At the start of the crystal-growing process with such apparatus, the weight on the pulling rope consists only of the weight of the crystal holder and the small, so-called seed crystal from which the crystal is grown. On completion of the crystal-growing process, however, the crystal and crystal holder will have a weight of from about 60 to 80 or more kg which must be supported by the rope pulling the crystal from the melt.
It was thought, therefore, that a sufficient strength for the rope corresponded to a breaking load of about 210 kg for a new rope. After some 15 to 20 uses, this drops drastically to a breaking load of about 100 kg which still seemed sufficient.
It has now been found, however, that the most critical part of the rope is the portion directly above the crystal holder at the start of a crystal-growing process. The crystal holder and the adjacent lower end of the rope to which it is fastened are then within the range of radiation from the melt and crucible and, possibly, even of the heater for the crucible. There, temperatures of over 1200.degree. C. can prevail. Temperature measurements made on a crystal holder in that critical position have shown that, in the case of a silicon melt having a temperature of 1450.degree. C., the temperature at the point of attachment of the rope is about 900.degree. C. This high temperature accounts for the fact that the breaking load of the rope is reduced by more than 50 percent in the cold state after repeated use, and it is to be expected that the breaking load is still lower in the hot state.
Thus, there is the danger that the rope will break at the crystal-holder end during the crystal-growing process due to thermal fatigue, with the crystal-holder then dropping into the melt. The resulting interruption of the crystal-growing process would be bad enough, but it can also displace enough of the melt onto the crucible heater to damage it, too. For example, the heater is often made of graphite which will then react violently with silicon to form silicon carbide. The latter has greater volume than the silicon, and the structural parts involved, for example the crucible, will therefore be shattered by stresses. The damage can easily range to thousands of dollars. There is also a safety hazard.
It has been sought to remedy the situation by making the crystal holder longer, thereby increasing the distance of the lower end of the rope from the maximum-temperature region. However, these efforts have never been more than moderately successful and entail the substantial drawback that, because of the increased length of the crystal holder, a corresponding portion of the so-called crystal-pulling path was lost, and only correspondingly shorter crystals could be grown. Increasing the overall height of the entire apparatus, which invariably is several times the length of the crystal, to compensate for the longer crystal holder often is out of the question because of the overall height of the available manufacturing bay. It has therefore been necessary up to now to replace the crystal-pulling rope after every 10 to 15 crystal-growing processes or so. While the resulting outage of the apparatus entails annoying production losses, these are put up with for reasons of greater-loss prevention and safety.